fix(native_comp): add libgccjit include dir during build stage

Also used existing `CFLAGS` and `LDFLAGS` environment variable values,
so a user can easily set these when running the build script to add more
paths. Previously they were set to explicit values ignoring any existing
value.

This might help resolve issues where libgccjit.h is not found as
reported in issue #20. Though I have not been able to reproduce this
myself, it seems adding the libgccjit's include dir has solved the issue
for some.

build-emacs-for-macos

@@ -200,14 +200,6 @@ class Build
     err 'This emacs source tree does not support native-comp'
   end
 
-  def gcc_library_paths
-    @gcc_library_paths ||= [
-      gcc_info.lib_dir,
-      gcc_info.darwin_lib_dir,
-      gcc_info.libgccjit_lib_dir
-    ]
-  end
-
   def autogen
     FileUtils.cd(source_dir) do
       if File.exist?('autogen/copy_autogen')
@@ -219,10 +211,10 @@ class Build
   end
 
   def compile_source(source)
-    target = "#{source}/nextstep"
-    emacs_app = "#{target}/Emacs.app"
+    target = File.join(source, 'nextstep')
+    emacs_app = File.join(target, 'Emacs.app')
 
-    if File.exist?("#{target}/Emacs.app")
+    if File.exist?(emacs_app)
       info 'Emacs.app already exists in ' \
            "\"#{target.gsub(root_dir + '/', '')}\", attempting to use."
       return emacs_app
@@ -240,20 +232,27 @@ class Build
 
         ENV['CFLAGS'] = [
           "-I#{File.join(gcc_info.root_dir, 'include')}",
+          "-I#{File.join(gcc_info.libgccjit_root_dir, 'include')}",
           '-O2',
-          '-march=native'
+          '-march=native',
+          ENV['CFLAGS']
         ].compact.join(' ')
 
         ENV['LDFLAGS'] = [
-          gcc_library_paths.map { |path| "-L#{path}" },
+          "-L#{gcc_info.lib_dir}",
+          "-L#{gcc_info.darwin_lib_dir}",
+          "-L#{gcc_info.libgccjit_lib_dir}",
           "-I#{File.join(gcc_info.root_dir, 'include')}",
-          "-I#{File.join(gcc_info.libgccjit_root_dir, 'include')}"
-        ].flatten.compact.join(' ')
+          "-I#{File.join(gcc_info.libgccjit_root_dir, 'include')}",
+          ENV['LDFLAGS']
+        ].compact.join(' ')
 
         ENV['LIBRARY_PATH'] = [
-          gcc_library_paths,
+          gcc_info.lib_dir,
+          gcc_info.darwin_lib_dir,
+          gcc_info.libgccjit_lib_dir,
           ENV['LIBRARY_PATH']
-        ].flatten.compact.join(':')
+        ].compact.join(':')
       end
 
       ENV['CC'] = 'clang'